The present invention relates generally to apparatus for welding together overlapped thermoplastic sheeting. Specifically, to a nozzle for such apparatus.
The use of geomembrane thermoplastic roof sealing material and industrial fabric-type material in sheet form has become increasingly popular. Such material, which generally comprises PVC, HDPE or the like, is typically provided in rolls. The material is laid down in strips and adjoining overlapped edges of the sheet material are welded by the application of heat to provide watertight seams. The resulting welded sheet material offers durable, long-lasting protection at relatively little expense and requiring little maintenance.
Various devices are known for welding the overlapped seams of thermoplastic material. Typically, such devices have a frame, wheels, hot air gun and welding nozzle. For example, Chitjian, U.S. Pat. No. 4,855,004, fully incorporated by reference and assigned to the assignee of the present invention, discloses such an apparatus.
Another example of a known seam welding device is disclosed in Stephenson et al., U.S. Pat. No. 4,440,588, entitled, "Hot Air Welder for Welding Roof Material", incorporated herein by reference. The disclosed apparatus includes a blower communicating with a heater assembly mounted on a chassis which includes a pressure wheel and a tracking wheel. The welder accommodates inside and outside seams.
Another example of a known hot-air seam welder marketed by the assignee as Model 2515 of its Cadillac.RTM. Products line includes a nozzle arrangement reproduced in FIG. 1 of the drawings accompanying this specification. Specifically, FIG. 1 shows a foot of the nozzle that is at the lower end of a nozzle body. Hot air passes through the nozzle body into the nozzle foot. The nozzle foot includes air slots across the width of the nozzle foot and spaced from the end thereof and vents on the upper and lower surfaces of the foot. The air slots and vents provide some preheating air to the thermoplastic roofing material as the foot passes between overlapped sheets. The main stream of the heated air is directed out the outlet of the foot. Heat is transferred to the sheets mostly through convection. There is relatively low radiant heat transfer by the nozzle surface.
As can be seen in FIG. 1, the nozzle foot 1 has a rounded shape similar to that of a quarter circle, in planar view, having a radius 2. The nozzle foot in FIG. 1 is divided, for purposes of explanation, into two Areas: Area 3 and Area 4. Air passes into the foot via the opening 5 positioned across the top of the foot. However, due to the rounded wall at the back of the foot 1, the airstream exhibits a river bend effect in which the stream concentrates along the rounded back wall. The result is a concentration of airflow in Area 4. A concentration of air flow in Area 4 is problematic because it may cause overheating of the equipment.
Another problem that occurs is that, while there is a concentration of airflow in Area 4, there is not enough heat in Area 3. Uneven distribution of air flow is undesirable because it causes uneven heating of the material. This problem is even more pronounced in nozzles that are 3 inches and wider. Clearly, for wide nozzles it is impossible to evenly heat the sheet in the width required (4 to 5 inches). Although there may be different combinations of holes to increase heat transfer, due to the low pressure inside the nozzle it is not possible to increase heat transfer substantially.
Also, the average welding speed is about 10 feet per minute, which translates into approximately 2 inches per second. The average distance from nozzle opening to the pinch rollers is a maximum 1 inch, but usually only about 1/2 to 3/4 of an inch. Thus, with the known devices, a user is only allowed approximately 0.2 to 0.3 seconds to properly heat the thermoplastic material.
It is desirable to increase the time for heating the thermoplastic material and to increase the radiant heat transfer across the nozzle foot. Further, it is desirable to provide an even output flow of heat across the nozzle foot output area between Areas 3 and 4. Uneven distribution of heat transfer across the weld area as well as in the output flow of hot air results in uneven seams between the thermoplastic sheets. Thus, there is room for improvement from the existing nozzles.